Pay attention: class made non-abstract. Reasoning: it allow to instantiate common event object:

GitHub.offline()
                .parseEventPayload(new StringReader(payload), GHEventPayload.class)

Why this may be needed: if you want to validate the X-Hub-Signature before handling the payload, you may need common information like repository to resolve the secret.
Possible workaround in case if the class is still abstract: make an empty subclass (in your local project, but in org.kohsuke.github package) and instantiate it.
Possible incompatibility: not expected.